Brendan Larder reported data from a test of over 125 clinical isolates with varying degrees of cross-resistance to indinavir, ritonavir, nelfinavir, and saquinavir in a recombinant phenotypic assay. These isolates ranged from having resistance to a single PI with a limited number of PI mutations to those highly co-resistant and containing 6 to 8 protease mutations.

The sample selection was from previously characterized PI resistant clinical isolates in Virco’s repository, representing a broad spectrum of PI susceptibility:

All viruses were sequenced to confirm mutation pattern before phenotypic analysis. They used the VircoGen method which uses ABI genotypic sequencing and the Virco Antivirogram phenotypic assay.

Results.  96/107 (90%) isolates showed susceptibilty to TPV. Eight (7.5%) showed intermediate levels of resistance to TPV. Three of 107 (3%) showed resistance with changes in the IC50 >10-fold. Of the 107 viruses cross-resistant to PIs the resistance to the other PIs ranged from 40 to 90 fold. TPV resistance was 2-fold. At least 85 isolates had >10-fold resistance to all four PIs. Among the highly cross-resistant isolates the more prevalent mutations were at 90, 71, 10, 48, 54, 82A, 84. Larder reported all of the samples resistant to single PIs (RTV, NFV, or SQV) remained fully TPV susceptible or were hypersensitive.

Viruses resistant only to NFV (n=10) or RTV (n=13) had no resistance to TPV, but the 5 saquinavir resistant viruses showed a 2.5 fold hypersenstivity to TPV. Larder reported 21 isolates with high-level resistance to the 4 PIs had 3.3 fold hypersensity to TPV. The ritonavir-only resistant viruses displayed mutations at 10, 46, 54, 71, 77, 82A, 82T, 84, and 90. The nelfinavir-only resistant viruses showed predominantly the D30N, which is the hallmark nelfinavir mutation. Almost 100% of these viruses had a D30N and 88 mutation. Also observed were mutations at 10,  36, and 71. The saquinavir mutations observed were a predominance of 48 and 90 and also 10, 36, 46, 54, 71, and 77.

Larder suggested hypersensitivity to TPV was associated with the presence of 48V/82A in a background of other secondary mutations. So far they have not been able to characterize a mutation profile for TPV.